The invention relates to high temperature solders for use in microelectronics that are destined for use in "harsh" environments.
As microelectronics are used in more rigorous or "harsh" applications, solder used in such applications has had to be improved to survive such conditions. The need for solders useful in such environments is increasing, with applications in automotive, avionics, and telecommunications arenas. Major difficulties with solders which are cycled repeatedly from low to high temperatures include adverse reactions with the substrate, and degradation and cracking due to thermal fatigue. Very high lead solders (98-100 wt. % lead) are known to be capable of good fatigue life, but are not used because of manufacturability problems, e.g., the wetting ability of the solders onto the required substrates is poor, and because alloy grain size coarsens with concomitant fatigue life degradation when exposed to high temperatures.
Current high-temperature solder technology uses, e.g., lead based alloys containing 10% tin (Sn) (i.e., "90-10" alloy) or 10% Sn and 2% silver (Ag) (i.e., "10-88-2"). These alloys have adequate fatigue life for most applications today, but are viewed to be inadequate for future applications in harsh environments; their thermal fatigue performance is adequate but not optimal; they suffer unacceptable loss of mechanical properties after prolonged exposure at elevated temperature (since there is reaction with substrates, producing brittle intermetallic phases and a weak solute depleted region near the substrate); they have a wide melting range, which is undesirable for use in electronics assembly; and a low solidus point, precluding use in applications at higher temperatures.
Cocks U.S. Pat. No. 5,120,498 teaches the addition of 0.01 to 10% lithium (Li) to improve the wettability of alloys on glass, containing at least two elements from the following: lead (Pb), tin, indium (In), cadmium (Cd), bismuth (Bi), mercury (Hg), antimony (Sb), silver, gold (Au) and gallium (Ga). However, these alloys contain substantial amounts of the aforesaid elements; furthermore the proportion of lead in the composition does not constitute the majority, i.e., less than about 95%, of the alloy. The presence of these elements lowers the melting point of these compositions, rendering them unsuitable for the highest temperature applications.
According to M. Schwartz in Brazing (ASM International, Metals Park, Ohio 1987), 0.2-3% Li is used in Ag-based brazing alloys to enhance wettability, and 0.2% Li is used in palladium (Pd)/nickel (Ni) alloys to enhance wettability. However, these alloys melt at several hundred degrees above the desirable range for solders.
According to W. Hofmann and H. Hanneman (Z. Metalkunde Vol. 20, pp. 47-49, 1938), the addition of sodium (Na) considerably delays the recrystallization of Pb.